Fix errors with automake 1.12

automake: warnings are treated as errors
/usr/share/automake-1.12/am/ltlibrary.am: warning: 'libumockdev-preload.la':
linking libtool libraries using a non-POSIX
/usr/share/automake-1.12/am/ltlibrary.am: archiver requires 'AM_PROG_AR' in
'configure.ac'
Makefile.am:24:   while processing Libtool library 'libumockdev.la'
Makefile.am:61: warning: 'VALAFLAGS' is a user variable, you should not
override it;
Makefile.am:61: use 'AM_VALAFLAGS' instead